1. Field of the Invention
This invention relates to a reversible thermosensitive coloring composition capable of developing and decolorizing a colored image repeatedly by utilizing a coloring reaction between an electron-donor coloring compound and an electron-acceptor compound and, more particularly, to a reversible thermosensitive coloring composition capable of stably maintaining the color development state and the decolorization state thereof at room temperature. This invention also relates to a reversible thermosensitive recording medium using the above-mentioned reversible thermosensitive coloring composition.
2. Discussion of the Background
Conventionally, thermosensitive recording media utilizing a coloring reaction between electron-donor coloring compounds (hereinafter, referred to as coloring agents) and electron-acceptor compounds (hereinafter, referred to as color developers) are widely known and have been employed in various fields, for instance, for use with terminal printers for computers, facsimile apparatus, automatic ticket vending apparatus, printers for scientific measuring instruments, and printers for CRT medical measuring instruments. However, such conventional thermosensitive recording media for use with the above-mentioned products do not have reversibility with respect to the coloring or decolorizing in image formation, so that the color development and the decolorization cannot be alternately performed repeatedly.
Among published patents, there are several proposals for thermosensitive recording media which can reversibly develop and decolorize or erase colored images utilizing a coloring reaction between coloring agents and color developers. For example, a thermosensitive recording medium using the combination of phloroglucinol and gallic acid as color developers is disclosed in Japanese Laid-Open Patent Application 60-193691. The images obtained by developing a color using gallic acid and phloroglucinol upon the application of heat thereto, is erased when coming into contact with water or aqueous vapor. In the case where this type of thermosensitive recording medium is employed, there are difficulties in imparting water-resisting properties to the recording medium and obtaining stable recording preservability. Furthermore, there is another problem in that a large image erasing apparatus is required to erase the displayed image on the above-mentioned recording medium.
In Japanese Laid-Open Patent Application 61-237684, a rewritable optical information recording medium which employs compounds such as phenolphthalein, thymolphthalein and bisphenol as color developers is disclosed. In the above optical information recording medium, colored images are formed by applying heat thereto and gradually decreasing the temperature thereof. The colored images can be decolorized or erased by applying heat to the recording medium at a temperature higher than the image developing temperature, and then rapidly cooling the recording medium. In the case of this optical information recording medium, the color developing and decolorizing steps are complicated and the contrast of the colored image is not satisfactory with some color remaining on the erased image which is obtained by erasing the displayed image.
In Japanese Laid-Open Patent Applications 62-140881, 62-138568, and 62-138556, thermosensitive recording media using a homogeneously dissolved composition of a coloring agent, a color developer and a carboxylic acid ester are disclosed. The above recording media can assume a completely colored state at a low temperature, a completely decolorized state at a high temperature, and can maintain the colored state or the decolorized state at a temperature midway between the above-mentioned low temperature and high temperature. When heat is applied to the recording media using a thermal head, a white image (decolorized image), which is similar to a photographic negative, is recorded on the colored background. Accordingly, the usage of above recording media is limited. It is also necessary that the temperature of the recording media be maintained within a specific range in order to preserve the recorded image on the recording media.
In Japanese Laid-Open Patent Applications 2-1882914 and 2-188293, there are disclosed a thermosensitive recording medium utilizing a salt of gallic acid and a higher aliphatic amine, and a thermosensitive recording medium utilizing a salt of a bis(hydroxyphenyl)acetic acid or butyric acid and a higher aliphatic amine. These salts have a reversible color developing and decolorizing function. With this type of recording medium, a colored image can be developed in a specific temperature range with the application of heat thereto, and can be decolorized or erased by applying heat thereto at a higher temperature than the above-mentioned specific temperature range. However, since the color developing effect and the decolorizing effect competitively occur, it is difficult to thermally control these effects by changing the temperature of the recording medium. Therefore, it is difficult to obtain a stable image contrast.
As mentioned above, the conventional reversible thermosensitive recording media utilizing the coloring reaction between a coloring agent and a color developer have many problems and are unsatisfactory for use in practice. In particular, a multiple colored image on a conventional reversible thermosensitive recording medium is completely unsatisfactory.
The inventors of the present invention have studied the coloring reactions between a variety of coloring agents and color developers. As a result, it is found that a coloring composition which comprises as a color developer a compound with a specific structure can perform stable color development and decolorization repeatedly as disclosed in Japanese Laid-Open Patent Application 03-355078.
The above-mentioned coloring composition contains a coloring agent and a color developer with a long-chain aliphatic group in its molecule. This coloring composition assumes a color development state by temporarily heating to a color development initiation temperature at which the coloring agent and the color developer are fused to develop a colored image. The colored image on the coloring composition is decolorized when temporarily heated to a temperature lower than the color development initiation temperature. This coloring composition can perform remarkably stable color development and decolorization in comparison with the above-mentioned conventional reversible thermosensitive coloring compositions, and therefore, a recording material which employs this coloring composition is capable of forming and erasing images easily by use of a generally used heat source such as a thermal head or heat roller. However, there are problems still remaining to be solved with respect to the image density after decolorization, the decolorization temperature range wherein the coloring composition assumes a decolorized state, and the decolorization speed, to obtain a coloring composition capable of yielding high quality images for practical use and to prepare a convenient reversible thermosensitive recording medium using such a coloring composition.